1. Field of the Invention
This invention relates to a method of geophysical prospecting which improves the subsurface area imaged by Vertical Seismic Profiling (VSP) data. Specifically, the invention uses surface multiples, which have previously been regarded as noise, to provide a migrated image of a larger portion of the subsurface.
2. Description of the Related Art
In surface seismic exploration, energy imparted into the earth by a seismic source reflects and scatters from subsurface geophysical features and is recorded by a multiplicity of surface positioned receivers. This process is repeated numerous times, using source and receiver configurations which may either form a line (2-D acquisition) or cover an area (3-D acquisition). The acquired data are processed to produce an image of the reflectors in the subsurface using a procedure known as migration. The produced image is then used in prospect evaluation and development. Prospect evaluation and development specifically includes using the determined geometry of subsurface traps to establish the volume of recoverable reserves, and the drilling of additional exploration, evaluation and development wells based on the image.
Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface impedance contrasts. The obtained image often suffers in spatial accuracy, resolution and coherence due to the long and complicated travel paths between source, reflector, and receiver.
In vertical seismic profiling, seismic signals are recorded using detectors in a wellbore to record the energy generated by the activation of seismic sources at the surface. This results in the geometry such as that shown in FIG. 1 and is discussed further below. It is well recognized that traditional migration of upgoing primary reflections produces a VSP data image of only a very narrow conical zone around a borehole with the cone tip centered at the shallowest receiver in a borehole. This is schematically illustrated in FIG. 1 wherein exemplary reflected 129a, 129b and 129c are shown. To extend this limited imaging zone, Yu and Schuster (2004) used a cross-correlation method to migrate the free-surface related multiples, and showed an improved image coverage for a 2D VSP dataset. Jiang et. al. (2005) employed an interferometric imaging principle to migrate the multiples and obtained an extended image coverage comparable to a common depth point (CDP) image area obtained by a surface seismic survey. Both cross-correlation and interferometric imaging methods are less sensitive to near-surface static or velocity errors, which makes them attractive to migration of multiples in those VSP data sets with complex near-surface conditions or severe near-surface statics problems.
However, both cross-correlation and interferometric imaging methods just migrate a single scalar receiver component of the recorded data, although the VSP data is generally recorded using a three-component downhole receiver and the multiples recorded by borehole receivers have the nature of three-component (3C) vector wavefields. The migration of all the three components simultaneously instead of a single scalar component can enhance or improve the image quality of the multiple wave arrivals.
In addition, the cross-correlation and interferometric imaging method are expensive for migrating the multiples from a large 3D VSP data set, due to their extensive and intensive cross-correlation calculations involved in the migration.